The present invention relates to the process control industry. More particularly, the present invention relates to remote seals and capillary tubes utilized in conjunction with pressure transmitters.
Pressure transmitters are used to remotely monitor the pressure of a process fluid. Remote seals, or remote diaphragm assemblies, are often used to distance the pressure transmitter from hazardous measurement environments, or for linking the pressure transmitter with inconveniently located process fluids. For example, remote seals are often used with corrosive or high temperature process fluids. In those situations, a remote seal having a diaphragm assembly and a capillary tube can be used to relate the pressure transmitter to the process fluid while the pressure transmitter is located a safe distance away. The capillary tube may extend tens of meters in order to couple the pressure transmitter with the process fluid. The remote seal communicates hydraulically with the process fluid through a thin, flexible diaphragm, which is used to isolate the process fluid from a fill fluid used in the capillary tube. As the diaphragm flexes, the incompressible fill fluid translates the pressure change through the capillary tube to a diaphragm located in the pressure transmitter. Deflection of the pressure transmitter diaphragm is transmitted through a similar method to a pressure sensor, which produces a signal relating to the pressure of the process fluid.
Remote seal hydraulic systems are filled at the factory with a precise amount of fill fluid and sealed. System performance is correlated with the precise level of fill fluid and is degraded with any oil leakage or the presence of air in the fill fluid. For severe applications, such as high process temperature or high process vacuum service, specialized factory production methods are used to remove air from the fill fluid and permanently seal the hydraulic system. For many less severe applications, the loss of a minute amount of fill fluid oil is acceptable.
Capillary tubes and remote seal diaphragms can be compromised due to wear and tear, accidents, and maintenance procedures, thereby causing leakage of fill fluid. For example, they can be cut or crimped as they are routed to their remote locations and therefore need to be repaired or replaced. Current methods for connecting the capillary tube to the pressure transmitter and remote seal include the use of a threaded or welded connection. These connections are not repairable in the field and are themselves a source of fluid leaks.
As such, it is desirable for less severe applications to have a field repairable, replaceable, and upgradeable remote seal system whereby leakage is minimized during coupling and uncoupling of the components and during operation.